Method and apparatus for combining transponders on multiple satellites into virtual channels

ABSTRACT

A satellite communications system provides an information channel between remotely located transmitters and receivers. A virtual satellite system provides the same service, but divides the signal either in power or in data content into subchannels such that any particular signal is conducted to the intended receiver via a plurality of traditional satellite channels. The receiving terminal accepts the plurality of signals simultaneously from a possible plurality of satellites, combining the subchannels comprising the virtual channel into the original signal content as if conducted via a single channel. The receiving antenna system receives satellite subchannel signals from a plurality of directions using multiple antennas or a single antenna with multi-direction capability. Prior to signal combining, the receiver necessarily time-synchronizes the plurality of subchannels by introducing time delay in some channels before combining the subsignals into the original composite. A timing signal present in the virtual satellite system assists the receiver in determining the amount of delay to apply to each incoming signal. The timing signal is either a separate carrier or an additional modulation on the existing information-bearing carrier.

RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 10/267,602. U.S. patent application Ser. No.10/267,602 was filed on Oct. 8, 2002 as a continuation application ofU.S. patent application Ser. No. 09/723,877 and issued as U.S. Pat. No.7,039,119 on May 2, 2006. U.S. patent application Ser. No. 09/723,877was filed on Nov. 28, 2000 as a continuation application of U.S. patentapplication Ser. No. 09/243,910 and issued as U.S. Pat. No. 6,549,582 onApr. 15, 2003. U.S. patent application Ser. No. 09/243,910 was filed onFeb. 3, 1999 and issued as U.S. Pat. No. 6,154,501 on Nov. 28, 2000.U.S. Pat. No. 6,154,501 claims priority from U.S. ProvisionalApplication No. 60/073,619 filed on Feb. 4, 1998 and U.S. ProvisionalApplication No. 60/113,693 filed on Dec. 24, 1998.

BACKGROUND OF THE INVENTION

The invention relates to satellite communications systems generally, andmore particularly to satellite communication systems which divide thetransmitted signal, either in power or in content, to be synchronizedand recombined in the receiving terminal. This concept applies readilyto broadcast applications, but it not so limited.

The satellite industry has experienced a progression of performanceenhancements evidenced by increased transmit power capability ofsatellite transponders, improved low-noise amplifier (LNA)characteristics, and smaller receiving antennas. In satellite systemswith a large number of receiving stations, it is particularly importantto reduce the cost of each receiving unit and to design a system with asmall receiving antenna to meet installation and aesthetic requirements.The need for a small receiving antenna has motivated an increase intransponder power output in order to maintain an acceptablesignal-to-noise ratio (SNR) with the smaller antenna. As satellite usersmove from lower power transponders to higher power transponders, fallingdemand for the lower power transponders reduces the cost of their use.Receiving a signal from a lower power transponder with the smallerreceiving antenna size produces a received power at the LNA too low tomaintain SNR requirements. The present invention permits the receiver tocombine received signals from a plurality of transponders, possiblylocated on a plurality of satellites to enable again the use of lowerpower transponders, but with small receiving terminal antennas.

SUMMARY OF THE INVENTION

A satellite communications system includes a transmitting station thatdirects information-carrying signals toward an orbiting satellite, whichreceives the signals and in turn retransmits the signals on a differentfrequency band toward a plurality of receiving terminals. The satellitecontains a transponder which receives signals as a broad band offrequencies and retransmits them on another set of frequencies of equalbandwidth but shifted to another location in the spectrum.

The present invention has as its object a satellite communicationssystem including a transmitting facility that divides the signal into aplurality of subchannels directed toward a plurality of transponderslocated on one or more satellites and a receiving terminal that receivesthe subchannels, time-synchronizes the subchannels, and combines theminto a faithful replica of the original composite signal. The divisionof the signal into subchannels is carried out by one of two methods. Ina first division method, the source signal is replicated across theplurality of transponders. Hereinafter the first division method isreferred to as power-division. In a second division method, the contentof the source signal is represented by a set of distinct subsignals,each of which subsignals contains less information as the originalsignal, but said distinct subsignals can be conveniently recombined inthe receiver to reconstruct the original signal satisfactorily.Hereinafter this second division method is referred to ascontent-division.

In a system using power-division to create subchannels, the originatingtransmitter directs more than one identical signal to a plurality oftransponders, possibly located on a plurality of satellites. In saidsystem, transponders retransmit and the receiving antenna systemconducts all of the aforementioned signals into the receiving system.The receiving terminal provides means of time-synchronizing theplurality of received signals, adjusts the relative power level of theplurality of received signals to be approximately equal, and combinesthe signals into a composite via a signal adding process. Each of thesignals added contains both an information component and a random noisecomponent, such noise having been introduced primarily in the firstamplifier of the receiver, typically a low-noise block converter (LNB).Those skilled in the art know that the information component of eachsignal will be statistically correlated, but the noise components willbe statistically uncorrelated, both to each other and to the informationcomponent. Thus the information components will add linearly into thecomposite signal, that is in proportion to their number. The power inthe information component of the composite signal will then be inproportion to the square of the number of received signals being addedtogether. In contrast, the power in the noise component of the compositesignal will be in proportion to the number of received signals addedtogether. Thus the SNR of the composite signal is improved over the SNRof the individual subchannel signals by a factor of N in power, where Nis the number of channels added together to form the composite signal.The foregoing discussion assumes that the signal levels and noise levelsin each of the subchannel signals is identical.

In a real system, transmission characteristics will vary slightlybetween subchannels, signal and noise levels being slightly differentbetween subchannels, resulting in an SNR improvement ratio somewhat lessthan the factor of N described above. In any case, the receiver mayrequire automatic means of adjusting the power of each of the signals tobe added at the combining point so as to be approximately equal to eachother in level.

In a system using content-division to create subchannels, theoriginating transmitter directs distinct subsignals toward the pluralityof transponders, the subsignals being created in such a way as to permitconvenient reconstruction of the original signal at the receivingterminal. In an exemplary analog system, the original signal can bedivided into subband signals using a filter-bank process. If the filtersused satisfy quadrature-mirror properties, the subsignals can be addeddirectly to reproduce the original signal without phase distortion atthe boundary frequencies. If the analog signal contains a strongperiodic timing component (as does a television signal), this periodictiming component can be separated from the remainder of the signalbefore dividing the signal into subband components. Said timingcomponent could then be added back to each of the subband components toproduce subchannel signals with different frequency components, butcommon timing information. This strategy naturally provides timinginformation useful to facilitate the necessary time-resynchronizingprocess in the receiver.

As above, in a system using content-division to create subchannels, theoriginating transmitter directs distinct subsignals toward the pluralityof transponders, the subsignals being created in such a way as to permitconvenient reconstruction of the original signal at the receivingterminal. In an exemplary digital system, the original binary signal canbe divided into subchannel digital signals, each of which has a bit rateless than the original digital signal. The original digital signal canbe divided into subchannel digital signals in any number of ways. Twosimple exemplary digital subchannel strategies are as follows. A firstexemplary digital subchannel strategy is to direct each successive bitinto each subchannel sequentially. A second exemplary digital subchannelstrategy is to direct each fixed-size block of bits in the originalsignal to each successive subchannel sequentially. This second exemplarystrategy fits well with digital source signals that are organized in afixed-block-size structure in the original signal.

In the case that a plurality of satellites is used to conduct a set ofsubchannels from a transmitting station to a given receiving terminal,each subchannel will generally experience a different propagation delay.The instant invention provides means to determine the amount of time todelay each subchannel in order to combine them synchronously. The delayrequired for each received subchannel will in the general case differ.The present invention provides additional means to implement theaforedetermined delay for each subchannel independently.

The receiving terminal system, when activated for a particular virtualchannel, determines the relative delay between the subchannel signalsarriving at the receiver. This could be accomplished by correlating thesubchannel signals with each other at all possible delays expected in aparticular implementation of the system. As this process is very timeconsuming and source signal dependent, it is therefore subject to falsesynchronization and possible failure to synchronize at all, particularlyif the source signal does not contain enough timing information. Thepresent invention solves this problem by transmitting a timing signalalong with the original signal. Said timing signal arrives at thereceiving terminal via a plurality of propagation paths, eachexperiencing a different delay. The timing signal is added to thevirtual satellite system in such a way so as to be separable from theoriginal signal on each subchannel. The receiving terminal thencorrelates timing signals arriving on different subchannels to determinethe amount of relative propagation delay. All subchannel signals containcommon timing information to facilitate the correlation process. Thisguarantees that the subchannels can be processed and compared in a knownway to determine relative propagation delay.

The timing signal can be added to the virtual satellite channel usingone of two exemplary methods, but the instant invention is not solimited. A first exemplary method requires that a narrow bandwidthsignal be transmitted across each satellite in the virtual channel. Thenarrow band signal requires a small allocation of the availablespectrum, but provides a dedicated timing signal on each satelliteactively carrying virtual satellite channels. The narrow band timingsignal provides propagation delay information to virtual channelreceiving terminals having one or more subchannels on the satellite. Thetiming signal could consist of one or more of the following exemplarysignals, but the instant invention is not so limited. A first exemplarysignal is a carrier modulated digitally by a binary pseudorandom noisesequence. A second exemplary signal is a periodic pulse. The pulse couldbe time-dispersed prior to transmission to create a signal with improvedpeak to average waveform properties. The receiving terminal in thisexample would reverse the time-dispersal process to recover anarrow-time pulse. The time period of either exemplary signal abovedescribed, after which the signal repeats, would be longer than twicethe greatest expected delay difference between subchannels, thusfacilitating unambiguous determination of propagation delay.

A second exemplary method of incorporating a timing signal in thevirtual satellite system consists of adding a spread spectrum componentto each of the information-bearing subchannels in the system, and withinthe bandwidth of each subchannel. The magnitude of the spread spectrumtiming component is much lower than the information signal so as not toreduce the performance of the normal receiver demodulation process. Thespread spectrum signal is then despread in the receiving terminal,thereby increasing its magnitude above that of the information content.The increase in signal level is proportional to the processing gain.This process facilitates delay synchronization in the receiving terminaland has two advantages. A first advantage is that the second exemplarymethod does not increase the bandwidth requirements of the virtualchannel to accommodate a timing signal. A second advantage is that thefull bandwidth of the information channel is available to the timingsignal resulting in high resolution relative delay estimation.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic and block diagram illustrating the presentinvention.

FIG. 2 is a schematic and circuit block diagram of one embodiment of thepresent invention.

FIG. 3 is a schematic and circuit block diagram of another embodiment ofthe present invention.

FIG. 4 is a schematic and circuit block diagram of another embodiment ofthe present invention.

DESCRIPTION OF THE INVENTION

Referring now to the drawings in which like reference numerals indicatelike or corresponding elements over the several views, FIG. 1 shows anoverview of the satellite communications system consisting of subsystems12, 10, 16. Original signal 22 feeds subchannel divider 24 whichseparates the signal into a plurality of numbered subsignals. Theexemplary system of FIG. 1 shows the number of subsignals to be four,but the present invention is not so limited. Subchannel divider 24creates the subsignals by dividing original signal 22 employing one oftwo methods. A first method divides the signal on the basis of power. Inthis first method all the subchannel signals emerging from subchanneldivider 24 are identical. A second method divides the signal on thebasis of content. In this second method, each subchannel signal carriesat least some information that is not carried by the other subchannels.The information content may be mutually exclusive or may overlap betweensubchannels, but in any case the subchannel signals under the seconddivider method are not identical as in the first method. Each subchannelsignal feeds an uplink transmitter 26 a-26 d, each of which uplinktransmitters feeds a separate antenna 28 a-28 d, directing radiofrequency energy toward a plurality of orbiting satellites 14 a-14 d viapropagation paths 18 a-18 d. Uplink transmitters 26 a-26 d add timingsignal 23 to the signal to be transmitted either on a separate frequencyallocation or in the bandwidth of the information-bearing carrier.

The exemplary system of FIG. 1 shows the number of satellites used bythe system to be four, but the instant invention is not so limited. Eachsatellite 14 a-14 d receives a band of frequencies, amplifies thesignals received in that band, and retransmits the band at a differentlocation in the spectrum. Each of said satellites has a transmittingantenna pattern that includes receiving terminal system 16. Propagationpaths 20 a-20 d from each satellite 14 a-14 d to representativereceiving terminal 16 carry radio frequency energy from satellites 14a-14 d to the receiving terminal system 16. It should be understood thatalthough FIG. 1 depicts each uplink signal being carried by a differentsatellite, the present invention is not so limited. For example,transponders of satellites 14 a, 14 b could be collocated on the samesatellite. In this case, uplink transmitters 26 a, 26 b and uplinkantenna systems 28 a, 28 b could be combined, in addition to satellites14 a, 14 b representing the same satellite. Propagation paths 18 a, 20a, 18 b, 20 b in this case would be combined into single uplink anddownlink propagation paths. Receiving terminal system 16 incorporatesone of two antenna methods. A first method includes a plurality ofantenna components to receive the plurality of satellite signals 20 a-20d. A second method incorporates a multiple beam antenna. The exemplarysystem of FIG. 1 uses multiple beam antenna 30, but the presentinvention is not so limited. In either of the aforementioned receivingterminal antenna methods, the antenna subsystem produces a plurality ofoutput signals corresponding to the subchannel signals emerging fromsubchannel divider 24 in uplink system 12. In the exemplary system ofFIG. 1, each of the numbered signals emerging from multiple beam antenna30 correspond to similarly numbered signals emitted by subchanneldivider 24. This signal identity remains true whether satellites 14 a,14 b of FIG. 1 are distinct or represent the same satellite as indicatedin the foregoing description. The subchannel signals emitted by multiplebeam antenna 30 feed a plurality of tuners 32 which then drive aplurality of demodulators 34. A signal emerging from one of thedemodulators 34 then represents a version of the corresponding output ofsubchannel divider 24, but delayed in time in proportion to the sum ofthe lengths of the corresponding uplink and downlink propagation paths18 and 20. In receiving terminal 16, delay component 36 further delaysfirst-arriving signals such that all the subchannel components arrive atsubchannel combiner 38 at nearly the same time. Said combiner 38produces a reconstruction 40 of original signal 22. The method used insubchannel combiner 38 is consistent with and corresponds to the methodused to divide original signal 22 in subchannel divider 24.

(Digital Content-Division)

The instant invention uses one of three methods to affect the dividingand combining operations of subchannel divider 24 and subchannelcombiner 40. In each of said methods, subchannel divider 24 of FIG. 1feeds a plurality of uplink transmitters 26 a-28 b, but the signalsemerging from subchannel divider 24 are different in nature depending ofthe dividing and combining method used. In a first dividing andcombining method, original signal 22 is digital. In said first method,subchannel divider 24 divides said digital signal into lower data ratesubchannel signals with binary content that contains at least somemutually exclusive information. The division could be on a sequentialbit-by-bit basis, could be on a sequential frame-by-frame basis, and mayor may not relate to possible framing in the original digital signal.The exemplary receiving terminal 16 of FIG. 2 depicts a two-subchanneldigital receiving system where the radio frequency carriers feeding thedemodulators 36 a and 36 b are quaternary phase shift keying (QPSK)modulated signals, but the present invention is not so limited. Saidfigure further indicates the use of a multiple beam antenna 30, but thepresent invention is not so limited. Referring again to FIG. 2, multiplebeam antenna 30 emits first and second signals into first and secondtuners 32 a and 32 b. Each tuner shifts a band of higher frequencies toa band of lower frequencies of equal bandwidth such that receivercontroller 42 sets the center frequency of the higher band, but thelower band is fixed. Tuners 32 a, 32 b emit QPSK modulated signals at afrequency that the QPSK demodulators 36 a, 36 b expect to receive. Asthere are two subchannels in the example of FIG. 2, the data rate of thebinary information contained in these QPSK signals is approximately halfthe data rate of original signal 22. The respective outputs of QPSKdemodulators 36 a, 36 b emit signals to bit detectors 38 a, 38 b whichin turn produce streams of binary data corresponding to the subchanneldivision in uplink system 12. Delay operators synchronize the datastreams by introducing delay in the first-arriving binary stream suchthat there is a minimum of relative delay between the respective delayoperator outputs. Digital content combiner 48 reverses the contentdivision process of subchannel divider 24 so as to produce at its outputa faithful delayed replica 50 of original digital signal 22. Receivercontroller 42 of FIG. 2 responds to user input (not depicted) to selectthe transponders 14 to combine, subsequently emitting control signals tomultiple beam antenna 30 to direct its antenna patterns toward thesatellites containing selected transponders 14. Receiver controller 42also selects each tuner frequency consistent with the signals emittedfrom the selected transponder. Receiver controller 42 further processesinformation from timing signal correlator 44 to determine the correctsetting of delays 40 a, 40 b. Timing signal correlator 44 receives andtime-correlates tuner outputs 34. For a system with more than twosubchannels, correlator 44 processes tuner outputs in pairs to determinerelative delay between subchannels. Nonvolatile memory 46 containsparameters regarding the user-selected transponders to enable thecorrect setting of multiple beam antenna 30 and tuners 32.

(Digital Power-Division)

The instant invention can use a second method for transporting a digitalsignal across a virtual satellite channel. Referring to FIG. 3 whichdepicts an example of said second method which combines delayeddemodulator outputs from identical subchannels as described previouslyas power combining. Under the direction of receiver controller 42,multiple beam antenna 30 emits signals to tuners 32 a, 32 b whichtranslate variable transponder bands into a fixed band of frequenciesexpected by the QPSK demodulators 54. FIG. 3 depicts a receivingterminal using a multiple beam antenna, but the present invention is notso limited. FIG. 3 further depicts a receiving terminal with twosubchannels, but the instant invention is not limited to twosubchannels. The figure in addition shows the use of a QPSK modulationscheme, but the instant invention is not so limited. Subchannel signals52 emitted by tuners 32 contain identical digital informationtransmitted at the full rate of original signal 22. QPSK demodulators 54produce soft decision outputs I_(A) and Q_(A) for each subchannel. Sincethe total propagation delay for each subchannel is in general different,first-arriving soft decisions must be delayed in time by an amount suchthat soft decisions emitted by delays 56 emerge with nearly zerorelative delay between subchannels. Delays 56 digitize the analog softdecisions produced by demodulators 54, placing digitized results in afirst-in first-out (FIFO) buffer. Receiver controller 42 controls theamount of time delay in delays 56 with input from timing signalprocessor 44 and digital correlator 58. Timing signal processor 44analyzes input from tuner outputs 52 to determine the relative timedelay between subchannels. For systems using more than two subchannels,the timing signal processor would process subchannel tuner outputs inpairs. Since the subchannels of FIG. 3 result from use of an uplinksystem 12 using power division, delay outputs I_(B) and Q_(B) fromdelays 56 a, 56 b are correlated. This enables digital correlator 58 tocompare digitized soft decisions between subchannels and provideadditional information to receiver controller 42 about relativesubchannel delay at the bit level. Digital power combiner 66 processessynchronized I and Q soft decisions from all subchannels to produce asingle I and Q decision 68 for every set of soft decisions presented.For the case of QPSK modulation, each final decision from combiner 66produces two bits in digital output 68.

(Analog Division)

A third method for dividing and combining the original signal addressthe case that original signal 22 is analog in nature. Referring to FIG.4, receiver controller 42 directs multiple beam antenna 30 to point toselected transponder signals and directs tuners 32 a, 32 b to translatesaid transponder frequencies to a fixed band of frequencies expected bydemodulators 70 a, 70 b. The exemplary system of FIG. 4 divides thesignal into two subchannels, but the instant invention is not solimited. Demodulators 70 a, 70 b produce analog outputs signals whichare faithful replicas of the subchannel signals produced by subchanneldivider 24 in the uplink system 12. Said analog signal outputs ingeneral experience relative delay due to differing lengths of totalpropagation paths when using transponders on different satellites. Underdirection of receiver controller 42, analog delays 72 add delay tofirst-arriving subchannel signals so as to create outputs of analogdelays 72 which arrive at analog combiner 80 with near zero relativedelay. Analog delays 72 consist of a high quality analog-to-digitalconverter (A/D), a FIFO buffer, and a digital-to-analog (D/A) converter.Each delays 72 creates a time delay in proportion the instant size ofthe FIFO buffer contained therein. Delays 72 present output signals toanalog combiner 80 which represent faithful replicas of the subchannelsignals produced by subchannel divider 24 in the uplink-system 12. Thesesignals differ outputs of demodulators 70 in that they are now timesynchronized. FIG. 4 represents both signal division strategies,power-division and content-division. In the first case of power-dividedsubchannel signals, inputs to analog combiner 80 represent identicalsignals, differing only in distortion and noise added by satellitetransport. In a second case, time-synchronized content-dividedsubchannel signals arrive at analog combiner 80. Analog combiner 80creates output 82 most likely by a simple addition process, but is notso limited. In addition to producing combined output signal 82, analogcombiner 80 optionally provides a measure of time synchronization toreceiver controller 42 to improve the accuracy of time alignment bycontroller 42. As in first and second digital divider-combiner methods,timing signal correlator 44 provides relative subchannel delayinformation to receiver controller 42, which together with furtheroptional delay information from analog combiner 80 provides receivercontroller 42 with a basis to create estimates of relative delay betweensubchannels which in turn affects the setting of delays 72.

(Timing)

In first, second, and third divider-combiner methods, tuners 32 provideinformation to timing signal correlator 44 using one of two timingmethods. In a first timing method, receiver controller 42 adjusts tuners32 to receive timing signal 23 placed on all satellites withtransponders used by the virtual satellite system. In this first method,tuner adjustment is necessary as the timing signals are placed at afrequency assignment separate from the information-bearing transpondersignal. This out-of-band timing signal may be narrow-band in nature soas to conserve limited bandwidth on the satellite and reduce systemcost. In general, timing signal 23 is unrelated to theinformation-bearing transponder signal in either information content,modulation strategy, or data rate or frame rate in the case of digitaltransmission, but the present invention is not so limited. The timingsignal utilizes allocated bandwidth to enhance the resolution ofrelative subchannel delay estimation. Possibilities for the timingsignals include pseudorandom noise, tone ranging, and time-dispersedpulse, but the instant invention is not so limited. A good timing signalmust have a strong sharp cross-correlation with a time-shifted versionof itself and have minimum spurious correlations. The instant inventionincludes two timing signal processor methods. In a first timingprocessor method, timing signal correlator 44 correlates output signalsfrom tuners 32 at various relative delays until an acceptablecorrelation occurs indicating that the relative delay between thesubchannels has been reproduced in timing correlator 44. Receivercontroller 42 then sets analog delays 72 in accord with this measuredrelative delay to synchronize inputs to analog combiner 80. In the casethat there are more than two subchannels in the virtual satellitechannel, timing signal processor 44 compares subchannel signalspairwise. In a second timing processor method, timing signal correlator44 correlates the output from each tuner 32 with a stored version of theknown timing signal, or by processing the recovered timing signalthrough a process that will produce a periodic output in response to thetiming signal. One example of such a process is a matched filter, butthe present invention is not so limited. Once the delays 40, 56, 72 areadjusted to remove relative subchannel delay, tuners 32 are set toconduct the selected information-bearing transponder signals to therespective demodulators in FIG. 1, FIG. 2, FIG. 3.

In a second timing method, the timing signal is as wide in bandwidth asthe information-bearing transponder and resides in exactly the samebandwidth. In order to prevent distortion of the information signal, thetiming signal is greatly attenuated. In order to recover the attenuatedtiming signal, timing signal correlator 44 first processes the tuneroutputs through a linear system that creates a large processing gain toamplify the expected timing signal above the output created by thepresence of the uncorrelated information-bearing carrier. The instantinvention may use one of three exemplary processes to recover alow-level in-band timing signal, but the present invention is not solimited. In a first exemplary process the timing signal is atime-dispersed pulse with precise time dispersion introduced by asurface acoustic wave (SAW) filter in timing signal generator 23. Amatching SAW filter in receiving terminal 16 performs the inverse of thedispersion process, thus recovering the primary timing signal which is aperiodic narrow-time pulse. In a second exemplary process, the timingsignal is pseudorandom noise. Timing signal processor 44 then appliesspread spectrum techniques to recover the timing of the low-levelin-band timing signal. Upon timing signal acquisition, the correlatedtiming signal will experience a large process gain, but the uncorrelatedinformation carrier will remain at the same relative level. This enablestiming signal processor 44 to establish relative delay betweensubchannels, reporting results to receiver controller 42. A thirdexemplary timing process uses a multiple tone signal to establishtiming. The sine waves selected are harmonically related in such a wayas to create a signal with a relatively long period, but giving goodtime resolution with the presence of some high frequencies. A linearfilter at the selected frequencies recovers the timing signal in favorof the information carrier. Timing signal processor 44 then analyzesfiltered timing signals to establish relative time delay betweensubchannels.

In the case of the digital content-division receiver of FIG. 2, there istypically no correlation between the subchannels to provide feedback asto the accuracy of the delay settings of delays 40. This is afeedforward control system. Feedback is possible however in theexemplary systems of FIG. 3, FIG. 4. Outputs from delays 56 in thedigital power-division receiver of FIG. 3 are highly correlated. If thedelay setting is slightly in error, a local digital correlation revealsthe necessary small correction. Outputs from delays 72 in the analogreceiver of FIG. 4 are correlated to some extent depending on the natureof the analog division and the instant properties of the analog content.This provides optional feedback to receiver controller 42 to affectlocal timing corrections.

While several particular forms and variations thereof have beenillustrated and described, it will be apparent that variousmodifications can be made without departing from the spirit and scope ofthe invention. Accordingly it is not intended that the invention belimited, except by the appended claims.

1. A method carried out at a communication system receiver, the methodcomprising: receiving a plurality of propagated signals, each of thepropagated signals carrying information associated with a source signaland being received by a respective tuner as a respective subchannel, andeach of the propagated signals comprising an information signal and anassociated timing signal; correlating a respective output signal emittedfrom each of the tuners so as to measure a relative delay between thesubchannels; synchronizing the information signals by delaying at leastone information signal by a time delay in accord with the measuredrelative delay; and combining the information signals, at least one ofwhich is delayed, to recreate the source signal.
 2. The method of claim1, wherein each of the propagated signals contains the same informationsignal.
 3. The method of claim 2, wherein each of the tuners provides arespective information signal in the form of a quadrature basebandsignal.
 4. The method of claim 1, wherein delaying at least oneinformation signal comprises generating sample sequences of each of theinformation signals, and buffering the sample sequences.
 5. The methodof claim 1, wherein delaying at least one information signal comprisesimposing an analog delay on the at least one information signal.
 6. Themethod of claim 5, wherein the analog delay is implemented via ananalog-to-digital converter, a buffer, and a digital-to-analogconverter.
 7. The method of claim 1, wherein each of the associatedtiming signals comprises an out-of-band timing signal that is assigned afrequency separate from a frequency of the information signal associatedwith the timing signal.
 8. The method of claim 1, wherein each of theassociated timing signals comprises a timing signal selected from thegroup consisting of (i) a timing signal including pseudorandom noise,(ii) a timing signal comprising a carrier modulated by a binarypseudorandom noise sequence, and (iii) a timing signal including atime-dispersed pulse.
 9. The method of claim 1, wherein each of theassociated timing signals comprises a spread spectrum timing signal thatis as wide in bandwidth as the information signal associated with thetiming signal and that is within a bandwidth including the informationsignal associated with the timing signal.
 10. The method of claim 1,wherein correlating the respective output signal includes correlatingthe respective output signal with a stored version of a known timingsignal.
 11. The method of claim 1, wherein correlating the respectiveoutput signal includes correlating the respective output signal withanother of the output signals pairwise.
 12. The method of claim 1,further comprising: producing analog in-phase and quadrature-phase softdecisions for each of the emitted output signals; wherein delaying theat least one information signal includes converting the analog in-phaseand quadrature-phase soft decisions into digital in-phase and quadraturesoft decisions, and wherein combining the information signals includesproducing a single digital in-phase and quadrature soft decisions fromthe digital in-phase and quadrature soft decisions.
 13. The method ofclaim 12, further comprising: correlating the digital in-phase andquadrature soft decisions so as to provide bit-level delay informationto a controller that controls an amount of time to delay the at leastone information signal.
 14. The method of claim 1, wherein each of thepropagated signals contains less information than the source signal. 15.An apparatus comprising: a plurality of tuners, wherein each of thetuners is configured to receive a respective propagated signal as arespective subchannel, wherein each of the propagated signals carriesinformation associated with a source signal, and wherein each of thepropagated signals comprises an information signal and an associatedtiming signal; a plurality of demodulators, wherein each of thedemodulators is connected to a respective tuner of the plurality oftuners; a timing signal correlator that correlates a respective outputsignal emitted from each tuner so as to measure a relative delay betweenthe subchannels; a plurality of delay devices, wherein each of the delaydevices is connected to a respective demodulator of the plurality ofdemodulators, and wherein the plurality of delay devices are configuredto use the measured relative delay to synchronize the informationsignals, wherein synchronization of the information signals delays atleast one information signal by the measured relative delay; and acombiner that is connected to each of the delay devices, wherein thecombiner is configured to combine the information signals, at least oneof which is delayed, to recreate the source signal.
 16. The apparatus ofclaim 15, wherein each of the demodulators is configured to produce arespective analog signal, wherein each of the delay devices isconfigured to receive the analog signal produced by the demodulatorconnected to the delay device, and wherein delay of the at least oneinformation signal comprises an analog delay.
 17. The apparatus of claim16, wherein each of the delay devices comprises an analog-to-digitalconverter, a buffer, and a digital-to-analog converter.
 18. Theapparatus of claim 16, wherein the combiner comprises an analogcombiner.
 19. The apparatus of claim 18, further comprising: a receivercontroller that is connected to the plurality of delay devices, thetiming signal correlator, and the analog combiner, wherein the receivercontroller is configured to receive delay information from the analogcombiner and the timing signal correlator and to direct the plurality ofdelay devices to delay the at least one information signal.
 20. Theapparatus of claim 19, wherein the receiver controller is configured toestimate relative delay of the information signals based on the delayinformation from the analog combiner and the delay information from thetiming signal correlator, and wherein the estimates of relative delayare used to set the delay of the plurality of delay devices.
 21. Theapparatus of claim 15, wherein each of the demodulators comprises aquadrature demodulator, the apparatus further comprising: a plurality ofbit detectors, wherein a connection that connects each delay device to arespective demodulator include a respective bit detector, wherein, uponoperation, the quadrature demodulators emit signals to the bitdetectors, and the bit detectors, in turn, produce streams of binarydata that are provided to the plurality of delay devices, and whereindelay of the at least one information signal includes delaying at leastone of the streams of binary data provided to the plurality of delaydevices.
 22. The apparatus of claim 21, wherein each of the tuners isconfigured to provide a respective information signal in the form of ahalf-rate quaternary phase shift keying (QPSK) signal or a full-rateQPSK signal to a respective quadrature demodulator.
 23. The apparatus ofclaim 15, wherein each tuner is configured to provide to the demodulatorconnected to the tuner an output signal comprising the informationsignal of the propagated signal received at the tuner, wherein each ofthe information signals comprise identical digital information, whereineach demodulator comprises a respective quadrature demodulator, andwherein each quadrature demodulator produces respective analog in-phaseand quadrature-phase soft decision for an output signal provided fromthe tuner connected to the demodulator.
 24. The apparatus of claim 15,wherein each of the associated timing signals comprises an out-of-bandtiming signal that is assigned a frequency separate from a frequency ofthe information signal associated with the timing signal.
 25. Theapparatus of claim 15, further comprising: a multiple beam antenna thatis operatively coupled to the plurality of tuners, wherein thepropagated signals received at the plurality of tuners are received atthe plurality of tuners from the multiple beam antenna.